700 



EEPTILIA. 



Fig. 345. 



artery is merely a small twig given off from the aortic system of vessels, 

 through which semi-arterialized blood passes to the lungs, to be returned 

 in a still purer condition to the left auricle of the heart. 



(1961.) If the student has fully comprehended the permanent con- 

 dition of the blood-vessels as it exists in the perfect Reptile and in the 

 Perennibranchiate Amphibian, he will have little difficulty in under- 

 standing the changes which occur in the distribution of the vascular 

 system during the metamorphosis of the CADTJCIBRANCHIATA. 



(1962.) In the Salamander, when the lungs begin to be developed 

 and are coexistent with the branchial apparatus, the arrangement of 

 the circulating system is precisely similar to that described as being 

 permanent in the Perennibranchiata ; as may be seen by a reference to 

 the appended diagram, which would equally illustrate the distribution 

 of the blood-vessels in both cases. 



(1963.) In this early stage of the tadpole's life, the contraction of 

 the heart and bulbus arteriosus 

 drives the greater part of the 

 blood through the branchial veins 

 (fig. 345, a a a) to the gills, from 

 which it is returned in a purified 

 condition by the branchial veins 

 (///), which, by their union, at 

 length form the aorta, as in fishes. 

 At this period the pulmonary ar- 

 tery (6), which is very small, in 

 correspondence with the as yet 

 rudimentary condition of the lungs, 

 is merely a branch derived from 

 the aortic system, and reinforced 

 by a vessel (c) given off from the 

 bulbus arteriosus. The greater 

 proportion of the blood therefore 

 evidently goes to the branchiae, 

 and a very small part to the lungs. 



(1964.) The reader must, however, here remark that there are small 

 anastomosing vessels (fig. 345, e e e), uniting the branchial arteries with 

 the trunks of the branchial veins, and that these are situated just at the 

 roots of the gills, since these vessels become of the utmost importance 

 during the subsequent stages of the metamorphosis. 



(1965.) The branchias gradually become diminished in size, and a 

 smaller quantity of blood passes through them ; and as this goes on, the 

 vessels (a a a,fff) shrink in the same proportion. Meanwhile the 

 lungs are progressively more and more developed, and the pulmonary 

 artery (6) expands in an equal ratio. As the blood forces its way with 

 more difficulty through the branchiae, the anastomosing vessels (e e e) 



Course of the circulation in Proteus 

 anyuinus. (After Rusconi.) 



